1. Field of the Invention
The present invention relates to a rapid catalyst warm-up control device for rapidly warming up a catalyst used for purifying exhaust gas from an internal combustion engine.
2. Description of the Related Art
In recent years, vehicles having an internal combustion engine have been provided with a catalyst such as three way catalyst for purifying exhaust gas from the internal combustion engine. In such systems, since the exhaust gas purification ratio of the catalyst remains low until the catalyst is warmed up to active temperature after starting the engine, measures are taken to rapidly warm up the catalyst. This is done by performing rapid catalyst warm-up control until the catalyst reaches its active temperature after starting up the internal combustion engine.
An example of this rapid catalyst warm-up control is such that injection dither control is performed to increase/decrease, and thereby correct, the fuel injection quantity to switch the injection mode between rich injection in which the air-fuel ratio is rich and lean injection in which the air-fuel ratio is lean, for example, for every injection (every 180° CA in case of a 4-cylinder engine). By doing so, the internal combustion engine is caused to discharge alternately rich gas with a high concentration of HC and CO and lean gas with a high concentration of O2 so that the rich and lean gases are mixed in the catalyst to create an oxidation reaction of the rich components (HC and CO). Accordingly, the catalyst is warmed up efficiently from the inside thereof by the heat of such a reaction.
According to the conventional rapid catalyst warm-up control technology using the injection dither control, the oxidation reaction of the rich components is not accelerated enough in the catalyst during the period when the catalyst temperature is low, which is immediately after starting the engine. This leads to occurrence of a phenomenon of “slip-through of the rich components” or “pass-through of the rich components” in which the rich components contained in the rich gas are directly discharged by slipping through the catalyst, and this constitutes a factor of increasing the emission of exhaust gas during start-up of the engine.
To cope with this problem, Japanese Patent Laid-Open Publication No. Hei 9-88564 (1997) (page 2 and so on) discloses a rapid catalyst warm-up control technique. According to the technique, after starting the engine, an ignition timing retarding control is first performed to raise the temperature of the exhaust gas and to warm up the catalyst by the heat of the exhaust gas. Then as soon as the catalyst temperature is increased to a certain degree, the control is switched to the injection dither control to accelerate the oxidation reaction of rich components in the catalyst and to efficiently warm up the catalyst from the inside thereof by using the reaction heat thus obtained.
According to the rapid catalyst warm-up control technique disclosed in the above-mentioned publication, however, the injection dither control is started only after the catalyst temperature has been increased to a certain degree by the ignition timing retarding control, meaning that the start of the injection dither control is delayed, and the warm-up of the catalyst is also delayed by that much.